The present invention relates to processing of pumpable food products, and more particularly to processing systems and methods for deactivating organisms in pumpable food products or foodstuffs, which systems and methods extend the shelf life of such food products or foodstuffs. Even more particularly, the present invention relates to the prevention or reduction of electrode fouling and/or prevention or reduction of electrochemical reactions in very high strength electric field systems for deactivating organisms in pumpable food products.
As used herein the phrases "deactivating organisms, "deactivate organisms," "deactivation of organisms" and similar phrases refer to the killing or sterilization of living organisms such as bacteria, viruses, fungi, protozoa, parasites and the like.
Substantial technical effort has been directed to the preservation of perishable fluid food products such as milk products, natural fruit juices, liquid egg products, and pumpable meat products, such as ground beef or turkey. Such liquid food products may normally contain a wide variety of microorganisms, and are excellent culture media for such microorganisms.
Practical preservation methods which have found significant commercial application predominantly utilize heat treatment such as pasteurization to inactivate or reduce the microorganism population. For example, milk products are conventionally pasteurized at a minimum temperature of at least about 72.degree. C. for 15 seconds (or equivalent time/temperature relationship) to destroy pathogenic bacteria and most of the nonpathogenic organisms, with degradative enzyme systems also being partially or totally inactivated. However, products processed in this manner are still generally unsterile and have limited shelf life, even at refrigeration temperature. The shelf life of liquid foodstuffs may be substantially extended by higher heat treatment processes such as "ultra high pasteurization", or "ultra high temperature" ("UHT") treatment, at a temperature of 140.degree. C. for four seconds. These processes are used in conjunction with aseptic packaging to achieve complete destruction of all bacteria and spores within the food product, however, such heat treatment typically adversely affects the flavor of the food product, at least partially denatures its protein content or otherwise adversely affects desired properties of the fluid food product. Other approaches to liquid food preservation, which also have certain disadvantages, include the use of chemical additives or ionizing radiation.
The bactericidal effects of electric currents have also been investigated since the end of the 19th century, with various efforts having been made to utilize electrical currents for treating food products. Such efforts are described in U.S. Pat. Nos. 1,900,509, 2,428,328, 2,428,329 and 4,457,221 and German Patents 1,946,267 and 2,907,887, inter alia, all of which are incorporated herein by reference. The lethal effects of low-frequency alternating current with low electric field strength have been largely attributed to the formation of electrolytic chemical products from the application of current through direct contact electrodes, as well as ohmic heating produced by current flow through an electrically resistive medium. Unfortunately however, the electrolytic chemical products generated by low frequency, low strength electric field methods may be undesirable in fluid foodstuffs, and heating, as noted above, may also cause undesirable effects in the fluid foodstuffs.
As described in U.S. Pat. No. 3,594,115, incorporated herein by reference, lethal effects of high voltage arc discharges have also been attributed to electrohydraulic shock waves. The utilization of explosive arc discharges to produce microbiologically lethal shock waves has not found wide-spread application as it is not a very effective means for preserving edible liquid foodstuffs. In addition, such explosive arc discharges can produce undesirable chemical byproducts in the foodstuffs being treated.
More recently, the effect of strong electric fields (or very high strength electric fields) on microorganisms has been studied as a mechanism for reversibly or irreversibly increasing the permeability of the cell membrane of microorganisms and individual cells. The application of very high strength electric fields to reversibly increase the permeability of cells has been used to carry out cell fusion of living cells and to introduce normally excluded components into living cells. Very high strength electric fields in nonnutrient media can also have a direct irreversible lethal effect upon microorganisms with the rate of deactivation dependent upon the field strength above a critical field level and the duration of the applied very high strength electric field.
A pulsed field treatment apparatus, which uses very high strength electric field pulses of very short duration, to deactivate microorganisms in food products is shown in U.S. Pat. Nos. 5,235,905 (the '905 patent); and 5,048,404 (the '404 Patent), issued to Bushnell et al., and U.S. Pat. Nos. 4,838,154 (the '154 patent); and 4,695,472 (the '472 patent), issued to Dunn et al., all of which are incorporated herein by reference. Generally, in accordance with the these patents, methods and apparatuses are provided for preserving fluid foodstuffs (or pumpable foodstuffs), which are normally excellent bacteriological growth media. Such preservation is achieved by applying very high strength electric field pulses (of at least 5000 v/cm) of very short duration (of no more than about 100 microseconds) through all of the pumpable foodstuff.
By "pumpable," "liquid," or "fluid" "food product" or "foodstuff" is meant an edible, food product having a viscosity or extrusion capacity such that the food product may be forced to flow through a treatment zone, e.g., less than about 1000 poise. The products include extrudable products, such as doughs or meat emulsions such as hamburger; fluid products such as beverages, gravies, sauces, soups, and fluid dairy products such as milk; food-particulate containing food slurries such as stews; food-particulate containing soups, and cooked or uncooked vegetable or grain slurries; and gelatinous foods such as eggs and gelatins.
By "bacteriological growth medium" is meant that upon storage at a temperature in the range of 0.degree. C. to about 30.degree. C., the fluid foodstuff, with its indigenous microbiological population or when seeded with test organisms, will demonstrate an increase in biological content or activity as a function of time as detectable by direct microscopic counts, colony forming units on appropriate secondary media, metabolic end product analyses, biological dry or wet weight or other qualitative or quantitative analytical methodology for monitoring increase in biological activity or content. For example, under such conditions the microbiological population of a pumpable foodstuff which is a bacteriological growth medium may at least double over a time period of two days.
The compositions of typical fluid food products which are biological growth media, derived from "Nutritive Value of American Foods in Common Units", Agriculture Handbook No. 456 of the U.S. Department of Agriculture (1975), are as follows:
__________________________________________________________________________ FLUID FOODSTUFFS Carbo- Fluid Water Protein Fat hydrate Na K Food Product Wt % Wt % Wt % Wt % Wt % Wt % __________________________________________________________________________ Whole Milk (3.5% fat) 87.4 3.48 3.48 4.91 .05 .144 Yogurt** 89.0 3.40 1.68 5.22 .050 .142 Raw Orange Juice 88.3 .685 .20 10.0 .0008 .2 Grape Juice 82.9 .001 tr. .166 .0019 .115 Raw Lemon Juice 91.0 .41 .20 8.0 .0008 .14 Raw Grapefruit Juice 90.0 .48 .08 9.18 .0008 .16 Apple Juice 87.8 .08 tr. 11.9 .0008 .10 Raw Whole Eggs 73.7 12.88 11.50 .90 .12 .13 Fresh Egg Whites 87.6 10.88 .02 .79 .15 .14 Split Pea Soup* 70.7 6.99 2.60 16.99 .77 .22 Tomato Soup* 81.0 1.60 2.10 12.69 .79 .187 Tomato Catsup 68.6 2.0 .588 25.4 1.04 .362 Vegetable beef soup 91.9 2.08 .898 3.9 .427 .066 __________________________________________________________________________ *condensed-commercial **from partially skimmed milk
Very high strength electric fields may be applied by means of treatment cells of high-field-strength design, examples of which are described in detail by Bushnell et al. and Dunn et al. Basically, the foodstuff is, in practice, electrically interposed between a first electrode, and a second electrode. The very high strength electric field is generated between the first and second electrodes such that the very high strength electric field passes through the foodstuff, subjecting any microorganisms therein to the very high strength electric field. Generally, the second electrode consists of a grounded electrode, and a relatively higher or lower voltage potential is applied to the first electrode.
In the Bushnell et al. patents and the Dunn et al. patent, the pumpable fluid foodstuff is subjected to at least one very high strength electric field and current density electrical pulse, and at least a portion of the fluid foodstuff is subjected to a plurality of very high strength electric field and current density pulses, in a high-strength electric pulse treatment zone. In one processing technique, the liquid foodstuff is introduced into a treatment zone, or cell, between two electrodes which have a configuration adapted to produce a substantially uniform electric field thereinbetween without dielectric tracking or other breakdown. Very high strength electric field pulses are applied to the electrodes to subject the liquid foodstuff to multiple pulse treatment by the pulsed field apparatus. In order to generate the very high strength electric field pulses, the pulsed field apparatus employs, for example, a lumped transmission line circuit, a Blumlein transmission circuit and/or a capacitive discharge circuit. Alternatively, the Bushnell et al. patents describe the use of field reversal techniques in capacitive discharge systems (or pulse forming networks) to increase the effective potential across the treatment cell. For example, by applying a short electric field pulse of very high electric field strength (e.g., 20,000 volts per centimeter) across a treatment cell for a short period of time (e.g., 2 microseconds) of one polarity, followed by abrupt reversal of the applied potential within a short time period (e.g., 2 microseconds), an effective field approaching 40 kilovolts per centimeter is achieved across the cell.
If liquid foodstuff (i.e., pumpable foodstuff) is continuously introduced into the treatment zone to which very high strength electric field pulses are periodically applied, and fluid foodstuff is concomitantly withdrawn from the treatment zone, the rate of passage of the liquid foodstuff through the treatment zone can be coordinated with the pulse treatment rate so that all of the pumpable foodstuff is subjected to at least one very high strength electric field pulse within the treatment zone. The liquid foodstuff may be subjected to treatment in a sequential plurality of such treatment zones, or cells, as is described in more detail by Bushnell et al.
Problematically, in processing some food products, such as milk or rich protein solutions, using the apparatuses and/or methods described by Bushnell et al., and Dunn et al., or the like, a film of materials can collect, or agglomerate, on the first and/or second electrode. This film of materials can consist of proteins and/or other materials (referred to herein as a fouling agent or polluting agent) that are present in the milk, or other protein rich materials. The formation of the film, or fouling of the electrode(s), is an undesirable side-effect that is believed to be due to the electrophoretic concentration of charged molecules within a boundary layer of food product that is adjacent to the treatment electrode. It has been noted that, for example, when the food product consists of raw milk, the fouling occurs only on the anode (i.e., the electrode to which electrons flow); the cathode (i.e., the electrode from which electrons flow) remains relatively free of any film buildup or agglomeration. Unfortunately, this agglomeration of the fouling agent on the electrode(s) during extended processing periods can cause electrical breakdown in the cell, fouling or contamination of the system, and in some cases can even cause the flow of fluid food product to stop. For some products, significant fouling of the electrode (or electrodes) can occur after only a few minutes of system operating time. For other products the time before which the fouling of the electrode (or electrodes) becomes significant can be a few hours or longer.
One attempt to solve a similar problem--electrolysis--is shown in the Dunn et al. patents. In accordance with the teachings of these patents, the suggestion is made that the first and second electrodes can be constructed so as to prevent direct electrolysis of the fluid foodstuff upon application of a pulsed electric field thereto. That is, the electrodes may each employ an electrically conductive electrolysis electrode, an ion permeable membrane and an intermediate electrolyte, such that ionic electrical connection is made with the fluid foodstuff through the ion permeable membrane rather than by direct contact with the electrically conductive electrode. Problematically, however, such electrolysis electrodes do not address the problem of electrophoresis, and they require the use of costly and cumbersome additional components in the pulsed field treatment apparatus. Thus what is needed is a way to prevent electrophoretic agglomeration of the fouling agent on the electrodes.
The problem of electrolysis, also, cannot be overlooked. Electrolysis occurring at the electrodes within a food product poses a further problem: that of electrochemical effects in the food product. These electrochemical effects are undesirable side-effects that can cause chemical byproducts to form within the food product at the electrodes and/or cause other undesirable effects within the food product. The use of ion permeable membranes, as shown by Dunn et al., offers one possible solution to this problem, but requires the addition of the ion permeable membranes surrounding the electrodes, which increases cost, and complicates the design of the electrodes. Thus, what is needed is a simplified approach to preventing electrolysis in very high strength electric field systems for deactivating microorganisms in food products,
The term "electrophoresis" as used herein refers to the process by which charged particles (e.g., relatively large protein molecules) suspended in a solution, such as a fluid food product (i.e., a pumpable food product or a liquid food product), are moved through the solution through application of an electric field to the solution. Electrodes that are used to generate the electric field may or may not be placed into the solution. The electrodes themselves, i.e., the materials from which the electrodes are made, do not participate in electrophoresis other than to generate the electric field.
The term "electrolysis" as used herein refers to the decomposition of a chemical system caused by the passing of an electrical current through the system. Electrolysis includes decomposition of chemical agents into chemical components, electrodeposition or electroplating of metals, reduction of metals, and charging of electrochemical batteries.